This invention relates to an automatic transmission lockup control apparatus for use with an automotive vehicle including an engine coupled to an automatic transmission through a torque converter operable in a lockup mode where a mechanical connection is completed between its input and output members.
A lockup command is produced to operate the torque converter in the lockup mode under an opera ting condition specified as a function of vehicle speed and engine load (as inferred from throttle valve position). It is the current practice to improve fuel economy by producing the lockup command at less vehicle speed and less throttle valve position. Normally, the vehicle speed is set at a minimum value possible to supply the working oil from the oil pump in an amount sufficient to provide a complete lockup condition in the torque converter. When the working oil is degraded, however, its viscosity decreases to cause working oil leakage through the lockup control valve. As a result, a great degree of slippage occurs in the torque converter to cause wear of the lockup clutch and reduction in the efficiency of drive transmission of the torque converter resulting in fuel economy loss.